Hi there, hope you are enjoying BYC!
Having hatched hundreds of chicks.. I'm gonna gave to go with something else.. that does
NOT look like a chick embryo.. and ask more questions.
How old is the hen that was unwell and what breed? Did you ever determine the cause of her condition or what were the symptoms, please? What is fed on a regular basis including treats and supplements? How are the weather conditions, how many birds in how much space, any recent changes or additions, anything notable? Are your fertile eggs collected daily, usually? Did you check the hens abdomen for swelling, is she molting with pin feathers or anything like that?
Thin shell.. could be a hiccup of the shell gland from the reproductive system coming or going offline like for molt... in this instance, maybe whatever was dragging her down.
Are you willing to cut into that blob and see if it looks like cooked egg on the inside? If so.. then salpingitis would be suspect.. sometimes called lash egg..
https://the-chicken-chick.com/salpingitis-lash-eggs-in-backyard/
Another resource for egg quality concerns and POSSIBLE causes..
Common egg quality problems BYC
Hope you get some answers and your gal thrives!